Effect of magnetic field on eutectic carbide morphology and mechanical properties in electroslag remelted M2 high-speed steel

Abstract

65, 130, and 160 mT transverse static magnetic field (TSMF) were introduced into the electroslag remelting (ESR) process to investigate the evolution of eutectic carbide morphology and mechanical property of M2 high speed steel. The application of TSMF induces the homogenization of the temperature field and reduces local solidification time, thereby inhibiting the non-heterogeneous nucleation and the growth of eutectic carbides. According to the result of electron back scatter diffraction (EBSD), as TSMF is applied and magnetic flux density (MFD) increases, the orientation of carbides becomes increasingly diverse and discontinuous. The results indicate that the application of TSMF leads to the refinement and dispersion of carbides, with the effect becoming more pronounced as the MFD increases. It enhances the wear resistance and hardness of ingots. The wear resistance significantly improved, with the maximum wear depth decreasing by 26.2% (9.54 to 7.04 μm) and the total wear volume dropping by 20% (2.75 × 10^7 to 2.20 × 10^7 μm³). Concurrently, the material's hardness increased from HRC 49.9 to 55.4. The overall results reveal that the presence of TSMF is beneficial for eutectic carbide morphology, thus achieving considerable improvement in mechanical properties of M2 high-speed steel ingots.

1. Introduction

AISI M2 high-speed steel (HSS) is widely used in high-speed cutting tools due to its excellent toughness, wear resistance, and red hardness. These properties result from the incorporation of finely distributed carbides in the matrix. However, high carbon and alloying element content (over 15 wt%) often causes significant segregation and the formation of coarse, brittle eutectic carbides [1–2]. Boccalini and Goldenstein [3] observed that such carbides severely degrade the mechanical and service performance of M2 HSS. Therefore, controlling the morphology and size of eutectic carbides is essential for improving the overall properties of M2 HSS.

Numerous technological solutions have been proposed to address the issues of carbide coarsening and uneven distribution in HSS, including adjusting cooling rates [4], adding alloying elements [5], vacuum casting [2], additive manufacturing [6–7], cryogenic treatment [8–9], hot deformation [10], and heat treatment processes [11–12]. However, for large ingots, achieving rapid cooling is challenging. The addition of alloying elements may simultaneously reduce other properties; for example, while silicon can refine eutectic carbides, it also diminishes the red hardness of the steel [13]. Even though additively manufactured steels show high strength, their ductility is inferior to that of cast or forged steels, particularly in high-carbon steels [6,14]. Moreover, the high cost of additive manufacturing constrains large-scale industrial application. In addition, once large-scale network eutectic carbides form during solidification, they are difficult to eliminate or refine by hot deformation and heat treatment, severely impacting subsequent processing steps [15]. Therefore, it is preferable to control carbides during the carbide formation stage.

Enhancing the cooling rate during solidification is one of the most effective methods to reduce elemental segregation and modify carbides [16]. Due to the efficient heat dissipation of water-cooled molds, electroslag remelting (ESR) is considered effective in regulating segregation and microstructure in high-alloy steels such as M2 HSS. However, as the diameter of ESR ingots increases, the cooling capacity of water-cooled molds diminishes, leading to reduced cooling rates, increased melt pool depth, and prolonged solidification time [17], which may give rise to metallurgical quality concerns. To address these defects, researchers have studied the ESR process from multiple perspectives, including optimizing remelting parameters such as current optimization [18–19], slag composition adjustment [20], and vacuum melting [21]; improving equipment structures such as single-power-two-circuits ESR [22] and rotating electrodes [23–24]; investigating droplet behavior [25]; and optimizing alloy compositions through microalloying [26]. These studies have demonstrated that a relatively shallow and flat melt pool during ESR facilitates the refinement of microstructures and eutectic carbides. Recently, magnetically controlled electroslag remelting (MC-ESR) has been introduced [27], coupling an external magnetic field with remelting currents to further control solidification.